It is well recognized that the production of thin epitaxial films of silicon having abrupt and arbitrary dopant profiles is vital in device and circuit fabrication, and particularly in applications such as scaled-down bipolar and CMOS VLSI circuits and processes. In particular, such thin epitaxial layers are useful in order to reduce the dimensions of high performance integrated circuitry. However, the fabrication of such thin epitaxial films is not possible owing to physical phenomena implicit in materials preparatory techniques heretofore known, as described by H. Ogirima et al, J. Electrochemical Soc., 124, 903 (1977). Specifically, the thickness of a device layer deposited by silicon epitaxy has been fixed at values greater than the diffusion length of dopants out of the substrate on which the epitaxial layer is deposited. These dimensions can be on the order of a micron under typical high temperature processing conditions (T.gtoreq.1000.degree. C.).
In more detail, prior techniques for depositing epitaxial silicon, as for instance the techniques described by G. R. Srinivasan, J. Cryst. Growth 70, 201 (1984) require high processing temperatures. At these high temperatures dopants in the substrate on which the epitaxial layer is deposited can move into the epitaxial layer either by evaporation and redeposition from the gas phase (autodoping), or can move out from the substrate by solid-state diffusion. Still further, dopants intended to be introduced into the epitaxial silicon layer can move in that layer and can diffuse into the substrate. All of the prior art processes for providing epitaxial layers (except for single wafer physical vapor deposition methods such as molecular beam epitaxy) operate at sufficiently high temperatures that dopant redistribution can occur. Because of this, the thickness of the deposited epitaxial layer must be fixed at values greater than the diffusion length of dopants out of the substrate, which in turn means that the ultimate size of a device produced in the epitaxial layer cannot be reduced below this dimension.
Over the past decade, the deposition of homoepitaxial silicon films for technical applications has been performed in essentially the same manner. Typically, the process takes place at temperatures in excess of 1000.degree. C. (or involves a high temperature cycle to clean wafers prior to deposition), using a cold wall/hot susceptor deposition apparatus of the type described by G. R. Srinivasan in Solid-State Technology, 24, 101 (1981). Advances in this technique have reduced autodoping by a lowering of processing pressures, a factor which has allowed continued use of the process.
However, the fabrication of very thin epitaxial layers having abrupt transitions (several atomic widths) in dopant concentration between adjacent single crystal layers cannot be achieved by such prior art techniques.
A low temperature process will be required which is sufficient to produce device quality, reproducible epitaxial films having the necessary thinness for device miniaturization. Several classes of such techniques presently being developed are described by G. R. Srinivasan and B. S. Meyerson in the Electrochemical Society Softbound Proceedings Series, Pennington, N.J. (1985).
Various types of low pressure chemical vapor deposition (LPCVD) processing techniques are known in the art, but these are used to produce polycrystalline and amorphous silicon. Typical process pressures used in such techniques are in the range of 200-1000 mTorr. The source gas used in these reactors is typically silane, with a carrier gas such as hydrogen. However, hydrogen carrier gas has a certain contamination level of H.sub.2 O (typically in excess of 1 part per million (ppm)) when it reaches the process environment and, for this reason, processing is generally performed in an atmosphere containing partial pressures of .gtoreq.10.sup.-4 Torr water vapor and oxygen. The effect of water vapor and oxygen must be taken into account in order to provide epitaxial silicon, since the crystallographic perfection of the initial silicon surface upon which epitaxy is to take place is the determining factor in the quality of the resultant epitaxial layer. Systematic investigations have been done in the past to determine the optimum cleaning procedure for a silicon surface prior to its insertion into the deposition apparatus. For example, reference is made to F. Hottier et al, J. Cryst. Growth, 61, 245 (1983) for an analysis of the procedures. Additionally, the quality of the environment into which the substrates are introduced is important. Ghidini and F. W. Smith, J. Electrochemical Soc. 109, 1300 (1982) and ibid 131, 2924 (1984) have conducted basic surface investigations of the Si/H.sub.2 O/SiO.sub.2 and the Si/O.sub.2 /SiO.sub.2 equilibrium systems to determine the equilibrium conditions in which both oxygen and water vapor background are such that silicon is effectively etched by these species in order to favor the maintenance of an oxide-free silicon surface.
In prior epitaxial silicon processing conducted at p.gtoreq.10 Torr., partial pressures greater than about 10.sup.-4 Torr water vapor and oxygen were present. An oxide free silicon substrate surface is obtained in such a system only if the deposition temperatures remain above 1025.degree. C., in accordance with the data of Ghidini and Smith, described in the referenced articles hereinabove. In these prior systems, source purity requirements are quite stringent in order to be able to operate at process temperatures as low as 1025.degree. C., which is in itself a high temperature.
The present invention is an apparatus and process for achieving device quality epitaxial silicon films without the heretofore mentioned problems, and in particular is a technique for high density batch processing of multiple wafers to provide epitaxial silicon films thereon. In applicant's technique, the temperatures and pressures utilized are much less than those previously utilized and are such that the process is nonequilibrium in nature, i.e., growth kinetics rather than equilibrium thermodynamics govern the deposition process. A hot wall, isothermal CVD apparatus is used in which essentially no homogeneous (gas phase) pyrolysis of the source takes place in the residence time (less than 1 second)/gas temperature regime where the process is operated. Instead, heterogeneous chemistry, where reactions at the surface of the substrate occur, are important.
As will become more apparent later, the present apparatus and process provide the following results and features, not heretofore reported in the literature or elsewhere:
1. In-situ doped CVD silicon epilayers at temperatures .ltoreq.800.degree. C. PA1 2. Use of ultrahigh vacuum (UHV) in combination with a CVD apparatus, thermally driven PA1 3. Use of a hot wall, isothermal system for Si epitaxy at temperatures less than about 800.degree. C., with high throughput PA1 4. Deposition of epitaxial silicon layers at low temperatures where the epitaxial layers are equilivent or superior in electrical characteristics to all epitaxial layers heretofore made PA1 5. The use of deposition temperatures as low as approximately 550.degree. C. to provide single crystal Si epitaxial layers having low defect densities, without the use of any external energy (lasers, RF plasmas, . . . ) PA1 6. A process and apparatus for batch fabrication of single crystal, epitaxial Si layers on a plurality of substrates wherein an isotropic source gas bath is produced in a thermally driven CVD apparatus.
Accordingly, it is a primary object of the present invention to provide a method and apparatus for epitaxial deposition of silicon layers in a batch process.
It is another object of this invention to provide an apparatus and method for enabling low temperature epitaxy of silicon layers.
It is another object of this invention to provide a CVD apparatus including means for providing ultrahigh vacuum therein, wherein the total system base pressure is less than about 10.sup.-8 Torr.
It is another object of this invention to provide a method and apparatus for low pressure, low temperature fabrication of epitaxial, single crystal silicon layers.
It is another object of this invention to provide a method and apparatus for producing an isotropic gas bath from a silicon source during thermally driven chemical vapor deposition.
It is yet another object of this invention to provide a method and apparatus for gas phase vapor deposition of single crystal silicon in a thermally driven process.
It is another object of this invention to provide a method and apparatus for growing single crystal silicon epitaxially upon a plurality of substrates at temperatures less than 800.degree. C.
It is another object of this invention to provide a method and apparatus for hot wall isothermal epitaxy of silicon layers from a gas source of silicon, said epitaxial layers being grown on multiple substrates at temperatures less than about 800.degree. C.
It is another object of this invention to provide a method and apparatus for chemical vapor deposition of epitaxial silicon layers via a thermally driven process.
It is another object of this invention to provide a method and apparatus for uniform epitaxial deposition of single crystal silicon onto multiple substrates via a low temperature, low pressure thermally driven chemical vapor deposition process.
It is another object of this invention to provide an apparatus and method for avoiding gas phase depletion in a hot wall thermally driven CVD process for depositing single crystal epitaxial silicon layers.
It is another object of the present invention to provide an improved thermal technique for epitaxially depositing silicon at temperatures less than about 800.degree. C.
It is another object of this invention to provide a method and apparatus for thermally driven chemical vapor deposition of epitaxial silicon wherein the source gases have reduced purity requirements.
It is another object of this invention to provide a method and apparatus for thermally driven chemical vapor deposition of single crystal silicon films upon multiple substrates wherein these films exhibit significantly reduced impurity contamination.
It is another object of this invention to provide a method and apparatus for thermally driven chemical vapor deposition of uniform single crystal silicon films upon multiple substrates.
It is a further object of this invention to provide a method and apparatus for in-situ doping of silicon epitaxial layers to levels above theoretical limits set by solid solubility during chemical vapor deposition of these layers.